Two disks of identical mass but different radii (r and 2r) are spinning on frictionless bearings at the same angular speed ωo, but in opposite directions. The two disks are brought slowly together. The resulting frictional force between the surfaces eventually brings them to a common angular velocity.

(a) What is the magnitude of that final angular velocity in terms of ωo
[tex]\frac{3}{5}[/tex]ωo < - I know that's right.

(b) What is the change in rotational kinetic energy of the system? (Take K as the initial kinetic energy.) This is what I need help with.

I used Ke=Iω2, coupled the system, considered one direction negative and one positive, and got [tex]\frac{9}{50}[/tex]mr2ωo2, which is apparently wrong. . .

Two disks of identical mass but different radii (r and 2r) are spinning on frictionless bearings at the same angular speed ωo, but in opposite directions. The two disks are brought slowly together. The resulting frictional force between the surfaces eventually brings them to a common angular velocity.

(a) What is the magnitude of that final angular velocity in terms of ωo
[tex]\frac{3}{5}[/tex]ωo < - I know that's right.

(b) What is the change in rotational kinetic energy of the system? (Take K as the initial kinetic energy.) This is what I need help with.

I used Ke=Iω2, coupled the system, considered one direction negative and one positive, and got [tex]\frac{9}{50}[/tex]mr2ωo2, which is apparently wrong. . .

I have spent more than two and a half hours on this problem and have gotten nowhere. I give up. Thank you vela and collinsmark for your help.

You're really close; you're just not calculating the rotational masses correctly, probably just algebra mistakes. Once you clear that up, you'll get the right answer. It might be good to take a break and come back to it later. Errors you can't see right now may pop out clear as day later.

You're really close; you're just not calculating the rotational masses correctly, probably just algebra mistakes. Once you clear that up, you'll get the right answer. It might be good to take a break and come back to it later. Errors you can't see right now may pop out clear as day later.